1. Field of the invention
This invention relates to an active-matrix display device such as a liquid crystal display device and so on, particularly to an active-matrix display device using thin-film transistors as switching devices.
2. Description of the prior art
A conventional active-matrix liquid crystal display device using thin-film transistors (hereinafter abbreviated as TFTs, as necessary) reduces crosstalk between the picture elements by the incorporation of TFTs thereinto, and removes limitations of the number of scanning lines. This device, therefore, gives a display device a greater capacity and higher quality as compared with a display device of simple matrix type. Even such active-matrix liquid crystal display device using TFTs, however, may incur deterioration of its image quality such as a decrease of the image contrast caused by crosstalk, in the case where the off-characteristics of the TFTs are inadequate or the off-resistance of the liquid crystal is insufficient. Moreover, occurrences of internal offset voltage within the liquid crystal cells or the application of an asymmetric voltage to the liquid crystal may cause deterioration of its image quality such as flickering, low contrast, deviation of a potential between electrodes, or the like.
To eliminate these problems, as shown in FIG. 6, a conventional active-matrix display device connected with a parallel circuit of the corresponding picture element 1 and the corresponding capacitance 2. The capacitance 2 alleviates deterioration of the image quality caused by inadequate off-characteristics of TFTs 3 and insufficient off-resistance of the liquid crystal.
However, the conventional display device mentioned above cannot fully resolve the deterioration problem of the image quality caused by an internal offset voltage within the liquid crystal cells and an asymmetric voltage applied to the liquid crystal.